In recent years, experts have been emphasizing importance of objective assessment of human fatigue for preventing deaths caused by accidents and overwork in the automotive and occupational fields. Upon assessing such fatigue, it is important to make the assessment in real time in practical situations, and to notify the user to pay attention to his fatigue, instead of the conventional fatigue assessments made in laboratories. However, the conventional assessments methods were not non-invasive, non-restraint, or simple, making the methods difficult to use in practical assessments.
In response to this problem, a method of clarifying correlation between feature values obtained from human pulse wave signals and fatigue to assess fatigue from pulse waves as a human fatigue assessment device aiming for practical use (for example, see Patent Literature 1). FIGS. 25A and 25B are block diagrams illustrating configurations of conventional human fatigue assessment device according to Patent Literature 1. The device in Patent Literature 1 shall be described with reference to FIGS. 25A and 25B as follows.
As illustrated in FIG. 25A, when a pulse wave measuring unit 2501 measures a pulse wave signal, an accelerated plethysmogram calculating unit 2502 calculates accelerated plethysmogram from the measured pulse wave signal, extracts waveform component of the accelerated plethysmogram, and calculates peak values of the first wave (a wave) to the fifth wave (e wave). Next, an assessment unit 2504 assesses that the user is in fatigued state when the newly calculated peak value is small than a reference value of a peak value of the accelerated plethysmogram stored in a storage 2503. More specifically, Patent Literature 1 particularly focuses on the a wave among waveform components of the accelerated plethysmogram, and illustrates the correlation between a decrease in the reduced peak value of the a wave and fatigue.
Patent Literature 1 also discloses a configuration in which a chaos analysis unit 2505 is added between the accelerated plethysmogram calculating unit 2502 and an assessment unit 2507, as illustrated in FIG. 25B. The chaos analysis unit 2505 performs a chaos analysis on the accelerated plethysmogram calculated by the accelerated plethysmogram calculating unit 2502, and calculates a maximal Lyapunov exponent. Next, the assessment unit 2507 assesses that the user is in fatigued state when the newly calculated maximal Lyapunov exponent is smaller than a reference value of maximal Lyapunov exponent stored in the storage unit 2506. According to Patent Literature 1, the configuration described above allows a non-invasive assessment of fatigue.
In addition, a method of estimating a state of a user, such as tension or sleepiness based on activities in autonomic nerves calculated by using pulse information corresponding to heartbeat obtained by measuring a pulse wave signal of a driver through a pulse wave measuring unit embedded in a steering wheel and others (for example, see Patent Literature 2).
In the method proposed in Patent Literature 2, when the amount of sympathetic nerve activity increases and the amount of parasympathetic nerve activity decreases (that is, when the sympathetic nerves are dominant), the driver is determined to be “excited” including an irritated state or an excited state. Alternatively, when the amount of sympathetic nerve activity decreases and the amount of parasympathetic nerve increases (that is, when the parasympathetic nerves are dominant), the driver is determined to be “sleepy” including a very sleepy state or an exhausted state. Alternatively, when both the amount of sympathetic nerve activity and the amount of parasympathetic nerve activity increase, the driver is determined to be “sleepy (contradicting)” indicating that the driver is trying to overcome the sleepiness, and when both the amount of sympathetic nerve activity and the amount of parasympathetic nerve amount decrease, the driver is determined to be “depressed (contradicting)”; that is, in a depressed state.